1. Field of the Invention
The present invention relates to using transducers based on electromechanical polymers (EMP) layers; in particular, the present invention relates to use of such transducers to provide haptic response in keys of a thin profile keyboard.
2. Discussion of the Related Art
Transducers are devices that transform one form of energy to another form of energy. For example, a piezoelectric transducer transforms mechanical pressure into an electrical voltage. Thus, a user may use the piezoelectric transducer as a sensor of the mechanical pressure by measuring the output electrical voltage. Alternatively, some smart materials (e.g., piezoceramics and dielectric elastomers (DEAP)) deform proportionally in response to an electric field. An actuator may therefore be formed out of a transducer based on such a smart material. Actuation devices based on these smart materials do not require conventional gears, motors, and cables to enable precise articulation and control. These materials also have the advantage of being able to exactly replicate both the frequency and the magnitude of the input waveform in the output response, with switching time in the millisecond range.
For a smart material that has an elastic modulus Y, thickness t, width w, and electromechanical response (strain in plane direction) S1, the output vibration energy UV is given by the equation:UV=1/2YtwS12  (1)
DEAP elastomers are generally soft, having elastic moduli of about 1 MPa. Thus, a freestanding, high-quality DEAP film that is 20 micrometers (μm) thick or less is difficult to make. Also, a DEAP film provides a reasonable electromechanical response only when an electric field of 50 MV/m (V/μm) or greater is applied. Thus, a DEAP type actuator typically requires a driving voltage of 1,000 volts or more. Similarly, a DEAP type sensor typically requires a charging voltage of 1,000 volts or more. In a handheld consumer electronic device, whether as a sensor or as an actuator, such a high voltage poses safety and cost concerns. Furthermore, a DEAP elastomer has a low elastic modulus. As a result, to achieve the strong electrical signal output needed for a handheld device application requires too thick a film. The article, “Combined Driving Sensing Circuitry for Dielectric Elastomer Actuators in Mobile Applications,” by M. Matsek et al., published in Electroactive Polymer Actuators and Devices (EAPAD) 2011, Proc. Of SPIE vol. 7975, 797612, discloses providing sensor functions in dielectric elastomer stack actuators (DESA). U.S. Pat. No. 8,222,799 to Polyakov, entitled “Surface Deformation Electroactive Polymer Transducers,” also discloses sensor functions in dielectric elastomers.
Unlike a DEAP elastomer, a piezoceramic material can provide the required force output under low electric voltage. Piezoelectric materials are crystalline materials that become electrically charged under mechanical stress. Converse to the piezoelectric effect is dimensional change as a result of imposition of an electric field. In certain piezoelectric materials, such as lead zirconate titanate (PZT), the electric field-induced dimensional change can be up to 0.1%. Such piezoelectric effect occurs only in certain crystalline materials having a special type of crystal symmetry. For example, of the thirty-two classes of crystals, twenty-one classes are non-centrosymmetric (i.e., not having a center of symmetry), and of these twenty-one classes, twenty classes exhibit direct piezoelectricity. Examples of piezoelectric materials include quartz, certain ceramic materials, biological matter such as bone, DNA and various proteins, polymers such as polyvinylidene fluoride (PVDF) and polyvinylidene fluoride-co-trifluoroethylene [P(VDF-TrFE)]. For further information, see, for example, the article “Piezoelectric Transducer Materials”, by H. JAFFE and D. A. BERLINCOURT, published on pages 1372-1386 of PROCEEDINGS OF THE IEEE, VOL. 53, No. 10, October, 1965.
The strain of a piezoelectric device is linearly proportional to the applied electric field E:S1˜E  (2)
As illustrated in equation (2), when used in an actuator device, a piezoelectric material generates a negative strain (i.e., shortens) under a negative polarity electric field, and a positive strain (i.e., elongates) under a positive electric field. However, piezoceramic materials are generally too brittle to withstand a shock load, such as that encountered when the device is dropped.
Piezoceramics and dielectric elastomers change capacitance in response to a mechanical deformation, and thus may be used as pressure sensors. However, as mentioned above, DEAP elastomers are generally soft, having elastic moduli of about 1 MPa. Thus, a freestanding, high-quality DEAP film that is 20 micrometers (μm) thick or less is difficult to make.
Unlike the piezoelectric materials that require a special type of crystal symmetry, some materials exhibit electrostrictive behavior, such as found in both amorphous (non-crystalline) and crystalline materials. “Electrostrictive” or “electrostrictor” refers to a strain behavior of a material under an electric field that is quadratically proportional to the electric field, as defined in equation (3)S1˜E2  (3)
Therefore, in contrast to a piezoelectric material, an electrostrictive actuator always generates positive strain, even under a negative polarity electric field (i.e., the electrostrictive actuator only elongates in the direction perpendicular to the imposed field), with an amplitude that is determined by the magnitude of the electric field and regardless of the polarity of the electric field. A description of some electrostrictive materials and their behavior may be found, for example, in the articles (a) “Giant Electrostriction and relaxor ferroelectric behavior in electron-irradiated poly(vinylidene fluoride-trifluoroethylene) copolymer”, by Q. M. Zhang, et al, published in Science 280:2101 (1998); (b) “High electromechanical responses in terpolymer of poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene)”, by F. Xia et al, published in Advanced Materials, 14:1574 (2002). These materials are based on electromechanical polymers. Some further examples of EMPs are described, for example, in U.S. Pat. Nos. 6,423,412, 6,605,246, and 6,787,238. Other examples include the EMPs whose compositions disclosed in pending U.S. patent application Ser. No. 13/384,196, filed on Jul. 15, 2009, and the EMPs which are blends of the P(VDF-TrFE) copolymer with the EMPs disclosed in the aforementioned U.S. Patents.
To achieve a substantially linear response and mechanical strains of, say, up to four (4) percent, in a longitudinal or transverse direction, the electrorestrictive EMPs discussed above requires an electric field intensity between 50 to 100 MV/m. In the prior art, to provide adequate mechanical strength and flexibility, the polymer films are at least 20 μm thick. As a result, an actuator based on such an electrostrictive EMP requires an input voltage of about 2000 volts. Such a voltage is typically not available in a mobile device.
Polyvinylidene difluoride (PVDF) and poly[(vinylidenefluoride-co-trifluoroethylene (P(VDF-TrFE)) are well-known ferroelectric sensor materials. However, these materials suffer from low strain, and thus perform poorly for many applications, such as keys on a keyboard.
FIG. 1 illustrates the basics of an exemplary EMP actuator 100 creating mechanical motion in response to an electrical stimulation. As shown in FIG. 1, EMP actuator 100 includes EMP layer 102, which may be itself consists of a number of EMP layers, is bonded to substrate 103. Substrate layer 103 is made of a thin, flexible material that is not compliant the longitudinal direction. During a quiescent state, i.e., without an electrical potential imposed across EMP layer 102, EMP actuator 100 is unstressed (FIG. 1a). When a DC electrical potential (i.e., 0 Hz) is imposed across EMP layer 102, EMP layer 102 elongates. As substrate 102 is not sensitive to the electrical potential, a large mechanical stress causes EMP actuator 100 to buckle, as shown in FIG. 1(b). With a non-zero driving frequency, vibrations may be created at different frequencies.
One area that EMPs find application is haptics. In this context, the term “haptics” refers to tactile user input actions. In a conventional keyboard, the mechanical, spring-loaded action of a pressed key and the associated audio “click” are haptic responses to the touch typist that a key has been successfully depressed. Similarly, a haptics-enabled touch screen may generate an immediate haptic feedback vibration when the key is activated by user input. The feedback vibration makes the virtual element displayed on the touch screen more physical and more realistic. In a portable device (e.g., a mobile telephone), a haptic feedback action can reduce both user input errors and stress, allow a higher input speed, and enable new forms of bi-directionally interactions, Haptics is particularly effective for keyboards that are used in noisy or visually distracting environments (e.g., a battlefield or a gaming environment). Haptics can reduce input error rates and improve response speed.
Recently proposed high-definition (HD) haptics may provide significantly more tactile information to a user, such as texture, speed, weight, hardness, and damping. HD haptics uses frequencies that may be varied between 50 Hz to 400 Hz to convey complex information, and to provide a richer, more useful and more accurate haptic response. Over this frequency range, a user can distinguish feedback forces of different frequencies and amplitudes. The feedback vibration is expected to be controlled by software. For a user to experience a strong feedback sensation, HD haptics in this frequency range, switching times (i.e., rise and fall time) between frequencies of 40 milliseconds (ms) or less are required. The ability to provide such HD feedback vibrations in the 50 Hz to 400 Hz band, however, is not currently available. In the prior art, a typical device having basic haptics has an output magnitude that varies with the frequency of the driving signal. Specifically, the typical device provides a greater output magnitude at a higher frequency from the same input driving amplitude. For example, if a haptic driving signal includes two equal-magnitude sine waves at two distinct frequencies, the output vibration would be a superposition of two sine waves of different magnitudes, with the magnitudes being directly proportional to the respective frequencies. Such a haptic response is not satisfactory. Therefore, a compact, low-cost, low-driving voltage, and robust HD haptics actuation device is needed.
Haptic responses need not be limited to 50 Hz to 400 Hz vibrations. At lower frequency, a mechanical pressure response may be appropriate. Vibrations in the acoustic range can be made audible. A haptic response that can be delivered in more than one mode of sensation (e.g., mechanical pressure, vibration, or audible sound) is termed “multimodal.”
Recently, the consumer electronics industry has been demanding very thin profile keyboards (e.g., 2-3 mm thick). For example, Microsoft Corporation introduced for its Surface tablet computer a keyboard which also serve as a protective cover of the tablet computer. FIG. 2 shows a side view of prior art thin profile keyboard 200. As shown in FIG. 2, keyboard 200 includes substrate or base layer 201, force-sensing resistor (FSR) layer 202 and cover layer 203. Base layer 201 provides protection and mechanical support for keyboard 200. FSR layer 202 is a sensing layer which is made of a material which resistance changes with an applied mechanical force on its surface (e.g., the pressure applied by a human finger, as illustrated in FIG. 2). Cover layer 203 provides protection to FSR layer 202. Cover layer 203 is thus made of a durable material. In addition, as the force of the user's finger is transmitted by cover layer 203 to FSR layer 202, cover layer 203 is made of a flexible material. Typically, base layer 201 is the most rigid layer of keyboard 200. However, without haptic feedback, such thin profile keyboards are not satisfactory to most users.